Method and apparatus for controlling supplemental heat in a heat pump system

ABSTRACT

A method and apparatus for controlling supplemental heat added to the air stream passing from an indoor coil to an air supply duct of a heat pump system, the heat pump system being of the type that includes an indoor thermostat having a first set point for initiating heat supplied by the indoor coil and a second set point for initiating additional heat supplied by supplemental heating means. The supplemental heating elements include an adjustable output heating element for heating air passing from the indoor coil to the air supply duct. The coil discharge temperature of the air stream heated by the indoor coil is determined at a position between the indoor coil and the supplemental heating means. The adjustable output heating element is selectively energized in response to the coil discharge temperature, independent of the second set point of the indoor thermostat.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional application of U.S. Ser. No. 09/012,542, filed Jan.23, 1998, now allowed, the entirety of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for controllingsupplemental heat in a heat pump, wherein a controllable heating elementis used in combination with the indoor fan coil to heat the supply air,and is controlled primarily based on the temperature of the air leavingthe indoor fan coil, substantially independent of the temperature sensedat the indoor thermostat. The temperature of the air leaving the indoorfan coil can be sensed directly or predicted based on the sensed outsideambient air temperature.

Heat pump systems use a refrigerant to carry thermal energy between arelatively hotter side of a circulation loop, where compression of therefrigerant by a compressor raises the temperature of the refrigerant,to a relatively cooler side of the loop at which the refrigerant isallowed to expand, causing a temperature drop. Thermal energy is addedto the refrigerant on one side of the loop and extracted from therefrigerant on the other side, due to the temperature differencesbetween the refrigerant and the indoor and outdoor air, respectively, tomake use of the outdoor air as a thermal energy source.

Heat pumps used in residential heating and cooling are bidirectional, inthat suitable valve and control arrangements selectively direct therefrigerant through indoor and outdoor heat exchangers so that theindoor heat exchanger is on the hot side of the refrigerant circulationloop for heating and on the cool side for cooling. A circulation fanpasses indoor air over the indoor heat exchanger and through ductsleading to the indoor space. Return ducts extract air from the indoorspace and bring the air back to the indoor heat exchanger. A fanlikewise passes ambient air over the outdoor heat exchanger, andreleases heat into the open air, or extracts available heat therefrom.

These types of heat pump systems can operate only if there is anadequate temperature difference between the refrigerant and the air atthe respective heat exchanger to maintain a transfer of thermal energy.For heating, the heat pump system is efficient provided the temperaturedifference between the air and the refrigerant is such that theavailable thermal energy is greater than the electrical energy needed tooperate the compressor and the respective fans. The temperaturedifference generally is sufficient for efficient cooling, even on hotdays. However, for heating when the outdoor air temperature is belowabout 25° F., the heat pump system may be unable to extract sufficientheat from the outdoor air to offset the loss of heat from the space dueto convection, conduction and radiation of heat from the structure tothe outdoors.

When the heat pump is unable to provide enough heat to the structure(i.e., the outdoor temperature is below the balance point between thebuilding load and the heat pump capacity) a supplemental heating elementis provided in the supply air duct downstream from the indoor heatexchanger/coil to supply the additional heat required to maintain thedesired indoor air temperature. Activation of the supplemental heatingis typically controlled by an indoor thermostat, by which the occupantsset a desired temperature to be maintained in the space by operation ofthe heating system.

Conventional heat pump control systems use atwo-stage-heat/one-stage-cool room thermostat. On a first call for heatfrom the thermostat, the heat pump compressor and fans are activated toextract heat outdoors and to release the heat indoors. The heat pumpsupplies air to the structure (typically at about 80° F.) until theindoor temperature reaches the thermostat set point (i.e., the first setpoint) and then is deactivated. If the heat loss of the structure isgreater than the capacity of the heat pump, which occurs when theoutdoor temperature drops, the indoor air temperature cannot be raisedby the heat pump to the desired temperature. The indoor temperature thuscontinues to drop.

The room thermostat has a second switching means that is operated at atemperature slightly lower than the desired temperature at which thefirst switching means is operated. Conventionally, when the roomtemperature falls to the second set point defined by the thermostat,power is supplied to the supplemental heating element. The supplementalheating element supplies the additional heat needed to bring the indoortemperature up to the second set point temperature (typically the supplyair is about 125° F.), and thereafter the heat pump works alone tosupply heat to the structure until the first set point temperature isreached.

As explained in U.S. Pat. No. 5,367,601, however, conventional two stageheat controls cause wide swings in the temperature of the supply airemitted into the structure by the heat pump system. Such widetemperature swings (e.g., 80° F. to 125° F.) are uncomfortable for theoccupants and adversely affect the efficiency of the heat pump system.In an attempt to improve occupant comfort, the '601 patent proposes acontrol system that provides a closer control on the operation of thesupplemental heating, by sensing the supply air temperature and thencontinuously controlling the on/off condition of the supplementalheating. While this proposal makes strides toward maintaining the supplyair temperature at a given level, it has at least two significantdrawbacks.

First, the supplemental heating is used only when there has been asecond call for heat from the indoor thermostat. This makes it moredifficult to maintain the air supply temperature at a constant,predetermined level, as supplemental heating is never energized duringfirst stage heating.

Secondly, the temperature sensor must be positioned in the air supplyduct of the building duct work by the technician installing the heatpump. Variations in the position of the sensor can lead to variations intemperature sensing accuracy, which in turn can lead to erroneouscontrol of the supplemental heating by the controller.

U.S. Pat. No. 4,141,408 also discloses control means for controllingsupplemental heating elements in a heat pump system. This patentproposes to use sensors positioned on the indoor coil to measure thetemperature of the air leaving the coil. The sensors are connected torelays that close to operate one or two fixed output heating elements.This system is unable to prevent wide swings in the air supplytemperature, because there is no means for operating the supplementalheating elements during first stage heating. There is also no means forprecisely controlling operation of the heating elements, in that theyare simply turned on and off in response to the temperatures sensed bythe sensors.

U.S. Pat. No. 5,332,028 also discloses a heat pump system havingsupplemental heat for application to the supply air during periods ofdefrost operation in order to avoid a "cold blow" condition while theheat pump is operating in the defrost mode. This patent proposes to turnon a supplemental heating element in response to the sensed temperatureof the supply air during defrost and responsively turn on additionalheat in stages when necessary to maintain the temperature level of thesupply air at a comfortable level during defrost. This system, however,also requires the installation technician to position the air supplytemperature sensor and thus suffers from the same drawback as discussedabove. And there is no means by which the supplemental heating elementsare controlled precisely in order to avoid the wide air supplytemperature swings mentioned above. Moreover, the supplemental heatingelements are not operated during first stage heating in order to insurea constant air supply temperature at all times.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to maintain thetemperature of the supply air emitted from a heat pump system at asubstantially constant level by providing for precise control of thesupplemental heating elements, while removing the possibility ofinstallation error with respect to the location of the temperaturesensor downstream of the indoor coil.

The present invention provides a method and apparatus for maintaining asubstantially constant supply air temperature in a heat pump system byproviding precise control of the supplemental heat supplied to the airstream passing from an indoor coil to an air supply duct of the heatpump system. The heat pump system is of the type that includes an indoorthermostat having a first set point for initiating heat supplied by theindoor coil and a second set point for initiating additional heatsupplied by supplemental heating elements. The method includes the stepsof providing an adjustable output heating element downstream of theindoor fan coil for heating air passing from the indoor coil to the airsupply duct. A microprocessor-based controller senses one of the outdoorair temperature and the coil discharge temperature of the air streamheated by the indoor coil at a position between the indoor coil and theadjustable output heating element, and then selectively energizes theadjustable output heating element in response to the sensed temperature,independent of the second set point of the indoor thermostat. If theadjustable output heating element alone cannot assist the indoor coil inmaintaining the air supply temperature at a predetermined basetemperature, then one or more fixed output heating elements can also beused.

The present invention is prefaced on the recognition by the inventorsthat, in order to maintain an air supply temperature at a predeterminedbase temperature of, say 105° F., it may sometimes become necessary toadd supplemental heat to the supply air when the heat pump system isoperating only in the first stage (i.e., in response to the first callfrom the indoor thermostat). The inventors also recognized that the useof fixed output heating elements, even if used during the first stage ofheat pump activity, often times supply too much heat to the supply air,thus causing the wide temperature swing problem experienced by priorsystems.

The invention overcomes this problem by using an adjustable outputheating element in combination with the indoor coil during the firststage of heat pump operation. The adjustable output heating element ispowered independent of the second set point of the indoor thermostat, inthat there need not be a call for supplemental heat from the thermostatbefore the adjustable output heating element is energized. In this way,the adjustable output heating element can be powered along with thefirst stage heating supplied by the indoor coil in order to maintain thesupply air at a substantially constant, predetermined base temperature(e.g., 105° F.). If the demand on the adjustable output heating elementexceeds its output capability, then the fixed output heating elementsalso can be energized one at a time in order to meet the building load.And, if the load causes the thermostat to call for supplemental heat,operation of the adjustable output heating element can be disengaged ifnecessary so that full power can be supplied to as many supplementalheating elements (including the adjustable output heating element) asnecessary to satisfy the second call from the thermostat.

The present invention also overcomes the sensor positioning problemsdiscussed above, by using a factory-installed sensor located at thedownstream side of the indoor air coil, or alternatively, using anoutdoor sensor. In either case, there is no calibration error introducedinto the system because there is no need for the installer to positionthe sensor at a precise location in the air supply duct work of thebuilding to be heated.

These and other objects of the present invention will be betterunderstood by reading the following detailed description in combinationwith the attached drawings of a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial representation of an indoor coil section of a heatpump system having the present invention incorporated therein;

FIG. 2 is a perspective view of the electric heating module portion of aheat pump system having the present invention incorporated therein; and

FIG. 3 is a graph showing heat pump capacity and building loadrequirements with respect to outdoor temperature and supply airtemperature.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, the invention is shown generally at 10 asincorporated into an indoor coil section 11 having a return air plenum12, a supply air plenum 13 and a blower motor assembly 14 for drawingthe air into the return air plenum 12 and supplying it back to the spaceto be conditioned by way of the supply air plenum 13. An indoor coil 16is disposed within the system and has refrigerant circulatedtherethrough for the purpose of cooling or heating the air passing overthe coil 16 as it is circulated through the system. The indoor coil 16acts as an evaporator in the cooling mode to remove heat from the indoorair and as a condenser in the heating mode to provide heat to the indoorair. During the defrost mode, the system switches from the heating modeto a cooling mode to allow the heat from the indoor air to betransferred by the refrigerant to the outdoor coil to facilitate thedefrosting thereof.

An electric heating module 17 is provided just downstream of the blowermotor assembly 14. Conventionally the electric resistance heatingelements in module 17 are energized to supplement the heat pump duringlow (e.g., less than 32° F.) outdoor temperature conditions. This moduleis also used during the defrost mode to heat the air being supplied tothe conditioned space while heat is removed from the return air for thepurpose of defrosting the outdoor coil. In accordance with the presentinvention, this module is also operated during the first stage of heatpump operation (i.e., when the indoor coil is usually acting alone toprovide the heated supply air). This aspect of the invention will beexplained later below in more detail.

A microprocessor-based controller 18 is provided to control the entireheat pump system in response to signals received from an indoorthermostat (not shown) and a temperature sensor 19, such as a thermistoror the like. Thermistor 19 functions to sense the temperature of the airleaving the indoor coil. Thermistor 19 can also be used to sense thetemperature of the outdoor air, and in both cases those temperaturesignals are provided to the controller 18 by way of leads 21 duringoperation of the heat pump.

The indoor coil 16 is connected to a standard closed loop refrigerationcircuit which includes a compressor 22, a 4-way valve 23, an outdoorcoil 24 with a fan 26 and expansion valves 27 and 28. The 4-way valve 23is selectively operated by the controller 18 to function in therespective cooling, heating, or defrost modes, with either the expansionvalve 28 functioning to meter the flow to the indoor coil 16 or theexpansion valve 27 functioning to meter the refrigerant flow to theoutdoor coil 24. The controller 18 can be applied to selectively operatethe compressor 22 and the fan 26 as well.

The electric heating module 17 is shown in greater detail in FIG. 2 toinclude a plurality of electric resistance heating elements 29 which areconnected to a pair of power leads 31 by way of a relay (not shown)controlled by controller 18. The heating elements 29 extend rearwardlyinto the supply air plenum 13 and are vertically supported by aplurality of support rods 32 as shown. Each of the heating elements ispreferably rated at 5 kW, although other rated elements can also beused. One of the heating elements is adjustable, in increments as low as100 W, from 0 up to 5 kW. The remaining elements, preferably up to threeadditional elements, are all fixed, preferably at the same output ratinglevel. FIG. 2 depicts only a two-element setup.

FIG. 3 shows a graph of outdoor temperature versus air supplytemperature, and includes plot HPC showing the heat pump capacity(determined by the parameters of the heat pump system itself) and plotBL showing the building heating requirements (building load). FIG. 3shows that heat pump capacity decreases and the building load increases,both substantially linearly, as the outdoor temperature decreases. Thebalance point is where the two lines cross. Conventionally, the firststage of a heat pump system is typically employed to serve the needs ofthe load at outdoor temperatures above the balance point, whereas secondstage heating (supplemental heating) is added to the air supply of thesystem at outdoor temperatures below the balance point. The balancepoint for the system depicted graphically in FIG. 3 is about 34° F.

In order to maintain a base air supply temperature of, say 105° F. (thehorizontal BT line in FIG. 3), the present invention selectivelycontrols the power supplied to the adjustable output heating elementbased on the following formula:

    kW=Constant×CFM×(T2-T1)

where T2 is the target base temperature (BT) of the supply air,

T1 is the temperature of the air leaving the indoor coil, and

CFM is the airflow through the system (which is known with some fanmodels and approximated with other fan models).

(The Constant simply assures reconciliation among the various units.)When T1 is sensed at the output of the indoor coil, that reading is useddirectly in the above formula. However, when sensing the outdoortemperature, T1 is predicted by extrapolation from the graph shown inFIG. 3. This can be done entirely within the controller 18 usingwell-known look up techniques.

In accordance with the invention and with reference to FIG. 3, thecontroller periodically calls upon sensor 19 for a temperature reading(T1). The controller then calculates the amount of kW power that must besupplied by the adjustable output heating element. If there has been acall from the indoor thermostat for first stage heat, but T1 equals T2,the system will cycle the first stage heat only, as depicted by the HPONLY section of the graph in FIG. 3. If, however, T1 is less than T2,the controller will calculate the amount of power to be supplied to theadjustable output heating element using the above formula, and thencontrol the power supplied to the adjustable output heating elementusing a solid state relay.

The preferred method of supplying power to the adjustable output heatingelement will now be explained by way of example. Say the calculatedpower requirement for the adjustable output heating element is 2 kW andthe full power rating of the adjustable output heating element is 5 kW.This means that 40% of the full power of the adjustable output heatingelement is required to raise T1 to the base temperature, T2(BT). Poweris supplied to the adjustable output heating element over a fixed numberof line cycles, say 100 line cycles for example. If the calculationdetermines that 40% power is required for the adjustable output heatingelement, then power will be switched on to that element for 40 linecycles and then switched off for 60 line cycles. This produces thenecessary 2 kW output from the adjustable output heating element. Thiscyclical application of power to the adjustable heating element isrepeated continuously for as long as the controller senses (via sensor19) that T1 is less than T2.

Preferably, the power to the adjustable output heating element ischanged incrementally, say in increments as low as 2% full power, inorder to allow precise control of the air supply temperature.Accordingly, if 40% power is called for the first time T1 is sensed, butT1 has decreased at the next reading cycle and the controller nowcalculates that 45% power (i.e., 2.25 kW) is needed to raise T1 to T2,then the power to the adjustable output heating element is increased by5% (i.e., continuously turned on and off for 45 and 55 line cycles,respectively) until T1 equals T2. Although increments of 2% can berealized using the present invention, increments of 5% full power areprobably as low as would be needed to deal with fluctuations in T1.

If the calculated power exceeds the rated output of the adjustableoutput heating element (e.g., 5 kW), then one of the additional fixedoutput heating elements (e.g., 5 kW each) will be energized by thecontroller and then the power to the adjustable output heating elementwill be changed continuously to meet the power demand in excess of 5 kW.

The adjustable output heating element preferably is switched on and offby a solid state relay while the remaining elements are switched on andoff using electromechanical relays. The solid state relay is driven by arelay driver circuit incorporated in controller 18. The solid staterelay has zero crossing circuitry which switches the adjustable outputheating element on and off only when the line cycle crosses zero volts.Thus, the on/off delay of onehalf line cycle limits the smallest on timefor the heating element to 2 line cycles. In the case of a heatingelement rated at 5 kW and operating on a 100 line cycle time base, thelowest power output would therefore be 100 W.

The partial lines F1, F2 and F3 in FIG. 3 that parallel the heat pumpcapacity line show the effect of energizing fixed 5 kW heating elements.The triangular shaded region R1 shows the added capacity as a result ofpowering the adjustable output heating element as described above. Thetriangular shaded region R2 shows the added capacity as a result ofpowering the adjustable output heating element while a first additionalfixed 5 kW heating element is energized by the controller. These regionsR1 and R2 are bounded by the BT temperature line (105° F. in FIG. 3).The intersection of the BT line with the BL and HPC lines dictates theoutdoor temperature range in which the adjustable output heating element(region R1) and, if necessary, one of the additional fixed outputheating elements (region R2) are energized cyclically with the firststage heating supplied by the indoor coil. To the left of theintersection of the BT line with the BL line, the heat pump capacity isso low that the system runs the indoor coil continuously and cycles theplurality of heating elements (including the adjustable output heatingelement as shown by shaded regions R3, R4 and R5) in order to meet theload demand of the building.

In accordance with the present invention, the air supply temperature canbe maintained at a substantially constant temperature, both during firstand second stage heating, by use of an adjustable output heating elementin combination with additional fixed output heating elements.

Additionally, the opportunity for installer-induced error can be avoidedby using a factory installed temperature sensor at the downstream sideof the indoor coil or an off-the-shelf outdoor temperature sensor.

While the present invention has been described with reference to aparticular preferred embodiment, it will be understood by those skilledin the art that various modifications and the like could be made theretowithout departing from the spirit and scope of the invention as definedin the following claims.

What is claimed is:
 1. A heat pump system, comprising:outdoor and indoor heat exchange coils with associated fans; a compressor; an expansion device; means for reversing the flow of refrigerant for purposes of selecting between heating, cooling, and defrost modes of operation; supplemental heating means for heating an air stream passing from the indoor coil to an air supply duct, said supplemental heating means comprising an adjustable output heating element; an indoor thermostat having a first set point for initiating heat supplied by the indoor coil and a second set point for initiating additional heat supplied by the supplemental heating means; means for determining the coil discharge temperature of the air stream heated by the indoor coil at a position between the indoor coil and the supplemental heating means; and control means for selectively energizing the adjustable output heating element in response to the coil discharge temperature, independent of the second set point of the indoor thermostat.
 2. The heat pump system of claim 1, wherein said means for determining the coil discharge temperature comprises temperature sensing means positioned between the indoor coil and the adjustable output heating element.
 3. The heat pump system of claim 1, wherein said means for determining the coil discharge temperature comprises outdoor temperature sensing means and the coil discharge temperature is predicted based on a reading from outdoor temperature sensing means.
 4. The heat pump system of claim 1, wherein said adjustable output heating element is energized when the coil discharge temperature decreases below a predetermined base temperature for the supply air.
 5. The heat pump system of claim 4, further comprising means for calculating the amount of power to be supplied to said adjustable output heating element based on the coil discharge temperature, and said control means selectively energizes said adjustable output heating element to raise the air supply temperature to the base temperature.
 6. The heat pump system of claim 5, wherein said adjustable output heating element has a maximum output, said calculating means calculates the amount of power to be supplied to said adjustable output heating element as a first percentage of the maximum amount, and said control means supplies power cyclically to said adjustable output heating element for a time-based percentage equal in magnitude to the first percentage.
 7. The heat pump system of claim 5, wherein said adjustable output heating element has a maximum output, and said control means incrementally changes the amount of power supplied to said adjustable output heating element based on the amount of power calculated by said calculating means.
 8. The heat pump system of claim 5, wherein said supplemental heating means further comprises at least one fixed output heating element and said control means energizes said fixed output heating element in addition to said adjustable output heating element when the amount of power calculated by said calculating means exceeds a predetermined level.
 9. The heat pump system of claim 2, wherein said temperature sensing means is a thermistor located adjacent said fan associated with said indoor coil. 